High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal

This protocol explains how to characterize the viscoelastic stretching properties of liquid crystals, and it helps to answer key questions about how to develop new photorheological materials. The main advantage of this method is that the rheological properties and relevant structural properties can measured under light stimuli in real time, allowing recording of the photorheological switching behavior. Before beginning the procedure, use a diamond-based glass cutter to cut glass substrates with average sizes of one-by-one centimeter, and wash the pieces at 38 or 42 kilohertz in an alkaline detergent.

Next rinse the substrates with 10 five-minute washes with sonication in fresh distilled water per wash, and subject the substrates to UV-ozone for at least 10 minutes. To add a planar alignment layer, use a pipette to dispense one milliliter of a polyamide planar alignment solution in 20-microliter droplets onto each cleaned glass substrate. Immediately use a spin coater to spin coat an approximately 20-nanometer thick alignment layer onto the substrates.

At the end of the spin, bake the coated glass substrates at 80 degrees Celsius for 60 minutes to remove the solvent, followed by curing for at least 60 minutes at 180 degrees Celsius. Then use a rayon cloth rubbing machine to rub the substrates at the appropriate parameters. Add 100 microliters of a photoreactive adhesive and 0.1 milligram of five-micrometer diameter glass particles onto a new glass substrate, and use the tip of a paperclip to mix the materials.

Transfer the mixed material to four corners of the glass substrate cell to adjust the cell gap, and use a low-pressure mercury vapor short arc lamp or a UV led to illuminate the cell with a 365-nanometer wavelength. After the illumination, place the cell onto a hot stage, and set the target temperature of the stage to heat the cell to a temperature above the isotropic liquid nematic phase transition. At the end of the phase transition, transfer the entire 0.2 to 10-microliter liquid crystal material onto one open surface of the cell.

Use a microspatula to push the materials towards the cell entrance to obtain contact between the liquid crystal material and the entrance of the cell. Then wait for the liquid crystal material to fill in the cell by capillary force. For texture characterization of the liquid crystal cells, place the samples onto the hot stage to control the sample temperature with a plus or minus 0.1 kelvin accuracy under a polarizing light microscope.

Use a digital color camera and a UV epi-illuminator to sequentially record the textures during cooling and heating with and without UV irradiation. For rheological measurement of the samples, first perform geometry inertia and zero gap calibrations within the software according to the manufacturer's instructions. Then weigh out 250 milligrams of the powdered CB6OABOBu sample.

Next load the sample onto the base quartz plate of the rheometer, and set the temperature of the sample chamber to a value above the isotropic-nematic phase transition point. Then set a gap value for approaching the measuring plate to the base quartz plate to sandwich the sample. Use paper lab wipes to trim any excess sample that is outside of the gap when the measuring plate stops at the trimming position.

To perform the measurement, irradiate the sample at 365 nanometers, measuring the photorheological switching of CB6OABOBu using the high-pressure mercury vapor short arc lamp. In the nematic phase, a uniaxial alignment of the molecules is realized. When decreasing the temperature to the twist-bend in darkness, a striped pattern forms, in which the stripes run parallel to the rubbing direction of the liquid crystal cell.

Further decreasing of the temperature leads to crystallization. Irradiation with UV light alters the conformation from a trans to a cis state, resulting in phase variation and thus texture variation, with the UV light transforming the striped texture to the uniaxial aligned state of the nematic phase when starting from the twist-bend phase. Turning off the UV light allows the molecules to relax and reenter the trans state, resulting in reformation of the striped texture of the twist-bend phase.

Measurement of the effective viscosity of the CB6OABOBu under various conditions reveals the temperature dependence of the effective shear viscosity. Here the shear stress dependence of the effective shear viscosity at different temperatures during the first and second runs can be observed. In this graph, the variation among the effective shear viscosity triggered by UV irradiation at different temperatures is shown.

And these graphs illustrate the switching curves of the effective shear viscosity in a log scale at two different temperatures. To obtain reliable data, it is crucial to calibrate the rheometer right before acquiring measurements. One idea for future studies on photosensitive dimers is to check a broadband dielectric spectra that could give insight into the molecular dynamics on the different phases and illumination conditions.